Overview
SPX (802.1BR) was introduced to the ICX system in FastIron 08.0.40. SPX includes a CB and a chain of PEs attached to the CB (for example, 1 CB with 4 PEs attached to it in a chain). The caveat here is that if a transit PE from the chain fails, the downlink PEs attached to it lose communication with the CB, thereby affecting traffic on the downlink PEs. An SPX PE provides a redundant connection for the PEs to communicate to the CB in the event of an upstream PE failure, with minimal impact on traffic.
In the figure, red arrows indicate the upstream traffic, and green arrows indicate the downstream traffic. The green X indicates the location of the logical block for data flow through the ring. The logical block is placed on the link that comes up last during the SPX (PE) ring formation. In this case, the link between PE 29 and PE 18 came up last. The logically blocked link in a PE ring remains up but intentionally blocks data traffic through it to prevent loops in the system.